Electrochromic devices, which usually comprise a substrate, a first transparent electrode layer, an electrochromic layer, an electrolyte layer, an ion storage layer, a second electrode layer and a second substrate, are used to control the light transmittance or reflectance of architectural windows or automobile room mirrors. The device uses the principle that an electrochromic material changes its color in response to external electric field. Recently, it has been reported that the electrochromic material can block the infrared as well as it can change colors in the visible light region, its application to be used to manufacture energy saving products has been spotlighted.
Electrochromic materials are classified into inorganic electrochromic materials and organic electrochromic materials. Representative inorganic electrochromic materials are WO3, NiOxHy, Nb2O5, V2O5, TiO2, MoO3, etc., and representative organic ones are polyaniline, polypyrrole, Prussian blue, etc.
The optical properties of the electrochromic materials can be changed through reduction or oxidation processes. The electrochromic materials are also classified into reduction coloring materials and oxidation coloring materials, depending on the coloring mechanism. By using a suitable reduction coloring material and an oxidation coloring material into the electrochromic layer and the ion storage layer, respectively or vice versa, the characteristics of an electrochromic device can be improved further by complementary effect.
Of the inorganic electrochromic materials, WO3, a typical reduction coloring material, performs color change through a reaction with ions and electrons, as illustrated in Scheme 1 below. The degree of coloring is determined by the amount of electric charge.WO3(transparent)+xe−+xM+MxWO3(deep blue)  [Scheme 1]
where x is a reaction coefficient, M is proton (H), lithium (Li), sodium (Na) or potassium (K) ion, typically lithium ion. The electrochromic effect results from the oxidation or reduction reaction of WO3 with the lithium ion. Such inorganic electrochromic material can be prepared by various methods, including vacuum evaporation, ion plating, deposition and sol-gel method.
Of the organic electrochromic materials, Prussian blue, a typical oxidation coloring material, performs color change as illustrated in Scheme 2 below,MFe4III[FeII(CN)6]3(blue)+e−+M+M2Fe4II[FeII(CN)6]3(transparent)Fe4III[FeII(CN)6]3(blue)+4e−+4M+M4Fe4II[FeII(CN)6]3(transparent)  [Scheme 2]
wherein M is proton (H), lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs) or ammonium (NH4) ion, most frequently lithium ion.
Typically, the organic electrochromic material is deposited by electroplating.
For Prussian blue, a method of applying an electric field between a substrate to be coated and a counter electrode for a long time in an aqueous solution of potassium ferrocyanide (K4Fe(CN)6) and ferric chloride (FeCl3) to form a coating film is known [U.S. Pat. No. 4,498,739; U.S. Pat. No. 4,818,352; U.S. Pat. No. 5,215,821]. However, the electroplating method has several fatal drawbacks—the time required for coating increases in proportion to the thickness of the coating film; and the coating process is discontinuous, and thus, is not suitable for large-scale production. Especially, when performing the coating on a large area, the size of the expensive counter electrode should be increased to obtain a uniformly coated film.
When a small-sized counter electrode is used, a non-uniformly coated film is obtained.